The control of human globin gene expression in erythroid cells involves trans-factors (substances active at distant locations in the genome), which have yet to be identified or clearly described. One experimental approach to their identification is to study the effects on globin gene expression of well-described tans-factors from tumor viruses. We have shown that the HTLV-I trans-factor tat-I stimulates both beta- and epsilon-promoters fused to a CAT gene, resulting in roughly 20-fold increase in CAT enzyme activity. In the case of beta-globin, only 185 bp of 5' flanking sequence is required for this effect. There is relatively little trans-activation when the globin promoter already has an SV40 enhancer in cis. Further studies will involve characterization of the tat-I induced trans-activation of globin promoters, including studies of mRNA levels and rate of transcription. While tat-I has been shown not to bind directly to DNA, we hypothesize that this trans-activation of globin genes involves interaction with other proteins that do bind to cellular DNA. Our ultimate objective is to identify such cellular proteins that interact with tat-1 to trans-activate globin genes. Study of such proteins would increase understanding of trans-activation of globin genes and may clarify the developmental regulation of globin gene expression in human erythroid cells.